There are staggered sessions starting every 30 mins or so throughout the day. Sessions are either 1.5 hours or 2.5 hours depending on the day. It is a bit of a jigsaw puzzle getting it to work! In the end, DH and I tried to limit the number of activities that both of us attended together so that we could always have one adult available to stay with the kids if needed. E.g., we booked separate massages on different days instead of a couples massage, separate drink tastings etc. Now with the recent Fantasy example where they had to shut down the kids club for several days on two cruises because of an outbreak, I think that strategy is even more wise.
Another suggestion: if you want to book an adult-only dinner do to the first day (when the club opens mid-afternoon) or a port day when you have an excursion booked. Then on your sea days, book the club for the morning to increase your odds of being able to book an additional session after your first one.
And even when you plan it perfectly, you do have to pack your patience. The one activity I booked for DH and I to attend at the same time was Remy brunch. When I got onboard and received the brunch reminder card at my stateroom, I saw they had moved my brunch time 45 minutes earlier than what I had booked online (presumably due to overbooking or maybe some concierge person wanted our time slot and they bumped us). I had planned the kids club time slot based on the original time. and it was now half an hour after the new brunch time they gave me. I had to go down to guest services to sort it out and eventually a manager was able to get me an override to let my three kids in the club in an earlier session that had an opening for just one kid.
It certainly is not as easy as pre-Covid, but compared with the hoops we had to jump through for other vacations and the top-notch service and experiences we had on the ship we still felt it was worth it. We also kind of liked the family togetherness that came from our kids having to do more stuff with us throughout the day. We did mini-golf, saw a LOT of family game shows, trivia, live music, character shows like "Sarge Says"--there is always something going on.